The present invention relates generally to the field of nondestructive testing. Specifically, the present invention relates to the technique of electronic shearography. More specifically, the present invention relates to the use of electronic shearography to detect defects in vehicle tires by animating shearograms produced while the tires undergo a varying stress continuum.
The technique of shearing interferometry, or shearography involves the interference of two laterally displaced images of the same object to form an interference image. Conventional shearographic methods require that a first interference image (or baseline image) be taken while the object is in an unstressed or first stressed condition, and another interference image be taken while the object is in a second stressed condition. Comparison of these two interference images (preferably by methods of image subtraction) reveals information about the strain concentrations and hence the integrity of the object in a single image called a shearogram. In particular, shearography has been shown to be useful to detect strain concentrations and hence defects in vehicle tires, especially retread vehicle tires.
In conventional electronic shearography, interference images are stored in a computer memory and are compared electronically to produce single static shearograms. Because all the data are processed electronically, the results of the analysis can be viewed in xe2x80x9creal timexe2x80x9d. xe2x80x9cReal timexe2x80x9d, as used in the prior art, refers to the ability to view the shearogram nearly instantaneously after the second interference image has been taken.
An apparatus and method for performing electronic shearography is described in U.S. Pat. No. 4,887,899 issued to Hung. The apparatus described in the cited patent produces an interference image by passing light, reflected from the test object, through a birefringent material and a polarizer. The birefringent material, which can be a calcite crystal splits a light ray, reflected from the object, into two rays, and the polarizer makes it possible for light rays reflected from a pair of points to interfere with each other. Thus, each point on the object generates two rays, and the result is an interference image formed by the optical interference of two laterally displaced images of the same object.
Prior to the developments disclosed in the Hung patent, the spatial frequency of the interference image produced in shearographic analysis was relatively high requiring the use of high resolution photographic film to record a useful interference image. The development disclosed in the Hung patent produces an interference image with a relatively low spatial frequency because the effective angles between the interfering rays are small. Therefore, the interference images can be recorded by a video camera, a video camera normally having much less resolving capability than a high density or high resolution photographic film. By storing an interference image of the object in its initial, unstressed condition, and by comparing that interference image, virtually instantaneously, by computer with another interference image taken under a different level of stress, a xe2x80x9creal timexe2x80x9d image or shearogram of the resultant strains on the object can be observed. Each point on the actual interference image is generated by the interference of light emanating from a pair of distinct points on the object. Therefore, each pixel of the video camera is illuminated by light reflected from those two points. If the overall illumination remains constant, then any variations in the pixel intensity, in the interference image, will be due only to changes in the phase relationship of the two points of light.
When the initial video image of the interference image is stored, an initial intensity for each pixel is recorded, as described above. If differential deformations occur in the object, such deformations will cause changes in the subsequent interference image. In particular, the intensity of a given pixel will change according to change in the phase relationship between the two rays of light, reflected from the two points on the object, which illuminate the pixel. The phase differences can be either positive changes, causing the pixel to become brighter or negative changes, causing the pixel to become darker. Whether the pixel becomes brighter or darker depends on the initial phase relationship and the direction of the change of phase. Due to the cyclic nature of phase interferences, as the deformation of the object continually increases, the intensity at a given pixel may pass through a complete cycle. That is, the intensity of the pixel might increase to a maximum (positive) difference, then return to the original intensity, and then continue to a maximum (negative) difference, and so on.
In systems of the prior art, a single shearogram is derived from two single static interference images taken at two distinct stress levels. The single shearogram is then viewed by an operator for analysis if multiple shearograms are taken, the analysis is done one shearogram at a time. Thus, the operator attendance time, required to perform a thorough stress analysis, is substantial. Further, a single shearogram may falsely show light features that appear to be defects (referred to as xe2x80x9cfalse positivesxe2x80x9d). These xe2x80x9cfalse positivesxe2x80x9d are caused by different reflective characteristics on the surface of the test object and appear as defects when a static shearogram is viewed. Further still, in a static shearogram some real defects may be xe2x80x9cwashed outxe2x80x9d and thus not visible (referred to as xe2x80x9cfalse negativesxe2x80x9d), at certain (particularly high) stress levels. These xe2x80x9cwashed outxe2x80x9d effects are caused by shearographic fringe lines that are not spatially separated enough to be visibly distinguishable and therefore appear to be aberrational light effects rather than real defects in the test object. Thus, a single static shearogram may contain inaccurate information with regards to the defects actually present. Furthermore, an operator having to analyze a large number of shearograms requires a large amount of operator attendance time.
There is a need and desire for an improved method of presentation of shearographic images that provide advantages over the prior art. There is also a need and desire for a method of presenting shearographic images that provide improved accuracy, shorter attendance times by an operator, and shorter overall cycle times for a test object. Further, there is a need and desire for a method of presenting shearographic images that reduce the undesirable effects of false negatives by preventing xe2x80x9cwash outxe2x80x9d of larger defects at high stress levels. Further still, there is a need and desire for a method of presenting shearographic images that allows real defects to be distinguished over light features that otherwise may be confused as defects, thereby minimizing false positives.
The present invention relates to an apparatus for performing electronic shearography on a test object. The apparatus includes a source of coherent electromagnetic radiation for illuminating the test object, and an optical element through which electromagnetic radiation reflected from the test object is transmitted forming an interference image. A detector converts the interference image into an electrical signal representative of the interference image. An animation device is coupled to the detector. The animation device receives the electrical signal representative of the interference image. The animation device retains image information derived from the electrical signals representative of the interference image at a predetermined frame rate. The animation device compares the retained interference image information with a baseline interference image to produce a shearogram image, and the animation device is adapted to play a series of sequential shearogram images. A display device is coupled to the animation device, providing visualization of the sequential shearogram images.
The present invention further relates to a method of analyzing a test object. The method includes directing coherent electromagnetic radiation onto a test object, providing electromagnetic radiation reflected from the test object to an optical shearing device, the optical shearing device creating an interference image, and directing the interference image, emerging from the shearing device, onto a detector. The method further includes capturing an electrical signal, communicated from the detector, in a capture device, the electrical signal being representative of the interference image, storing interference image information in a memory device communicated from the capture device and comparing interference image information stored in the memory device, to a stored interference image to produce a shearogram image. The method still further includes repeating the aforementioned steps at varying stress levels and displaying shearogram image information at a frame rate.
The present invention still further relates to an apparatus for performing electronic shearography on a tire undergoing varying states of stress. The apparatus includes a source of coherent electromagnetic radiation for illuminating the tire, a birefringent material through which electromagnetic radiation reflected from the tire is transmitted, and a polarizer through which electromagnetic radiation, emerging from the birefringent material, is transmitted, the birefringent material and the polarizer cooperating to form an interference image. The apparatus also includes a video camera, the video camera converting the interference image to an electrical signal and a video capture circuit coupled to the video camera, the capture circuit receiving the electrical signal from the camera, the electrical signal being representative of the interference image, the capture circuit retaining image information derived from the electrical signals representative of the interference image at a frame rate. Further, the apparatus includes a computer coupled to the capture circuit, the computer adapted to compare sequential interference images retained by the capture circuit to a baseline image to produce a shearogram image, the computer adapted to play the sequential shearogram images, and the computer including a display device coupled to the computer providing visualization of the sequential shearogram images and a memory device, coupled to the computer, the memory device being adapted to store the interference image information retained by the capture circuit.